The present invention relates to solid state, high gain differential amplifier circuitry and more particularly to such circuitry having an electronically controllable current and common mode voltage.
Many present day electronic systems such as those including filters could utilize high gain, high frequency differential amplifier circuitry having electronically controllable characteristics which can be manufactured in monolithic integrated circuit form and which has a wide enough bandwidth to handle from direct current (d.c.) to radio frequencies (RF). Fast acquisition demodulators, for instance, require active filters which can be electronically switched from a wide bandwidth to a narrow bandwidth. More specifically, such filter circuitry is required to provide the wide bandwidth until a desired input signal is acquired and then be switchable to the narrow bandwidth without losing the received signal which is then demodulated. Still other applications require filter circuitry having bandwidths that are continuously adjustable from 0 Hertz (Hz) to many Megahertz (MHz), in response to control signals or which can be switched from band reject to low pass operation, for instance.
Prior art circuits for use in filters having electronically adjustable bandwidths suitable for operation at audio frequencies (AF) are generally unsuitable for operation at RF because of the transport delay within such circuits. These circuits may have reactive networks connected in series with the signal path therein. Such networks provide signal delay that becomes more significant as the frequency of operation increases thus limiting the usable frequency range thereof.
Also, some prior art configurations tend to undesirably provide electrical transients or shifts in bias levels when switched from one bandwidth to another. These level shifts and transients can have an adverse effect of the linearity and dynamic range of such circuits. Although the foregoing circuits may be useful for many applications, they are not suitable for some sensitive, high frequency filter applications, for instance.
Because of their many inherent advantages, it is desired to use a differential amplifier configuration in monolithic form to solve the above and other problems. Some prior art monolithic differential amplifier circuits, however, include resistive loads connected to the collectors of the differentially connected transistors. Since the gain of a differential amplifier circuit varies directly with the load impedance, monolithic resistors which typically have low values do not provide sufficient gain for some sensitive applications. Other monolithic prior art differential amplifier configurations include PNP transistors connected to the collector electrodes of the differentially connected transistors to form differential-to-single ended converter circuits which are required to switch at the frequency of the dynamic signal being amplified. Unfortunately, it is difficult to manufacture high quality high frequency PNP transistors in monolithic integrated circuit form by commonly used present day processes. More specifically, such PNP transistors may have a unity gain cutoff frequency of about 20 MHz whereas the NPN transistors manufactured by such processes have a unity gain cutoff frequency of about 6 gigahertz (GHz). Also, the operating characteristics of these PNP transistors tend to vary with temperature, etc., thereby tending to adversely change the common mode voltage of the differential amplifier.
Direct coupled active loads often require that the common mode voltage of a differential amplifier remain substantially constant for a particular application. It is also desirable that the common mode voltage be adjustable to accommodate any one of a variety of direct coupled loads. Also, it is desirable for the main current of the differential amplifier to be adjustable to control gain, bandwidth and power dissipation.